The present invention is directed to an apparatus for continuously measuring the internal resistivity of a web of sheet material as the material is moved over a roller.
It is desirable to accurately measure the internal resistivity of a conducting layer sandwiched between insulating layers of sheet material while the sheet is moving through a machine at a relatively high speed.
With the internal resistivity known, it can be determined in advance whether a given sheet product, such as a photographic paper, will encounter electrostatic problems during sensitizing, finishing, or customer operations. Paper stock resistivity influences important electrostatic effects associated with moving webs and, therefore, maintaining internal resistivity within a limited range can prevent many electrostatic problems. For example, a low resistivity paper stock prevents the build up of a net charge on the surface of a moving web. If the charge did build up, the resulting electric fields could cause an air breakdown creating a spark or corona, which in turn would expose the emulsion layer on the photographic paper. In addition, low resistivity paper stock provides a means of preventing winding rolls from obtaining a high electrical potential which can result in external fields that can be hazardous not only to the product, but also to personnel. If the paper stock resistivity is too low, however, it can cause problems when the moving web is treated with high voltage sources, such as is used in a Corona Discharge Treatment (CDT). Excessive currents will propagate along the conducting moving web and can again be hazardous to personnel and product. In the case of extremely high paper resistivity, devices used to treat the web surface, may become ineffective.
Instrumentation is currently available for measuring the internal resistivity of a resin-coated sheet, but the measurement is accomplished off-line and is done normally several hours after the sample has been taken. During this time it is possible for the internal resistivity to change due to humidity conditions which, therefore, can lead to misleading results. In addition, if the internal resistivity is found to be either too high or too low, nothing can be done to correct the situation because the stock roll has already been sensitized.
A prior art patent of interest for its teaching of measuring resistivity in a moving web material is U.S. Pat. No. 2,870,405 entitled "Apparatus For Measuring Moisture in Web Coatings" by R. A. Wright et al. The aforementioned patent is assigned to Eastman Kodak Co., the assignee of the present application. The apparatus of that patent utilizes a roller having a plurality of radially projecting ribs extending parallel to the roller's long axis. Alternate ones of the ribs are electrically connected to ground through a slip ring and a measurement device, such as an ampmeter. Every other one of the ribs is connected to a d.c. source through a second slip ring. The circuit path from the d.c. source to ground is completed by the web material. The current flow through the resultant circuit is a function of the resistivity of the web material. Such an apparatus is sufficient for having a surface that is measuring webs of material if the whole web is conductive, but for materials which have the conductive layer sandwiched between layers of electrically insulating material, the apparatus will not work.
Another patent of interest is U.S. Pat. No. 2,942,243 entitled "Apparatus and Method For Detecting Abrupt Changes in Dielectric Sheet Material" by W. H. Huggins. In the apparatus of that patent, a sheet of dielectric material is passed between two electrodes with one of the electrodes being formed from a plurality of skids which yieldably engage the dielectric material. The apparatus functions not to measure resistance but to detect bumps, creases or other sudden changes in the thickness of the dielectric sheet material by creating a voltage pulse which is a.c. coupled to amplifiers that drive some form of alarm or indicating device.
Because the patented apparatus uses electrodes which make sliding contact with the surface of the material being monitored, the apparatus cannot be used to monitor webs of materials having a sensitized outer layer because the skids will physically harm the sensitized layer. In addition, the patented apparatus will not respond to slow changes in material thickness because it generates a voltage pulse by a quick, or transient change, in the spacing between the two electrodes.